Friday, March 30, 2012
During the next few weeks the four brightest planets -- Jupiter, Venus, Mars and Saturn -- will be visible during the night. These planets are brighter than the stars, with Venus being the brightest. This is a great time to go planet hunting.
Unlike planets, stars are so far from Earth that they will not noticeably change their relative positions in our lifetimes. But planets move among the stars (the word planet means wanderer).
Where do I find planets in the night sky?
First, we need a reference. We will use the "celestial equator," which is a projection of the Earth's equator into the sky. In Walla Walla, this is an arc that starts at the due east point on the horizon, reaches its highest point due south 44 degrees above the horizon, and then continues down to meet the horizon at the due west point.
If the Earth's rotation were perpendicular to its orbital plane, this is the path the sun would travel through the sky; but since the Earth's rotational axis is tipped by 23.5 degrees, the sun travels in a zone 23.5 degrees above and below the celestial equator. Since the orbital planes of all the other planets are very close to that of the earth, the planets are always close to this zone. This means that in Walla Walla, we never look north to see the planets.
Mercury's orbit is the closest to the sun (too close to see right now), followed by Venus, Earth and Mars. Jupiter and Saturn come next. The farther a planet is from the sun, the longer it takes to go around the sun and the slower it moves relative to the stars. This means, for example, that Venus moves faster than Saturn. Which planets we can see during a given night depend on both where each planet is in its orbit and where the Earth is in its orbit.
So where are the planets tonight? If we look west at about 8 p.m., we find the first two planets, Venus and Jupiter. The brightest planet about 35 degrees above the horizon is Venus, and Jupiter is just 14 degrees below it.
During the next few weeks Venus will stay in about the same position, while Jupiter will move away from Venus and closer to the horizon each night. Though they appear close in the sky, Venus and Jupiter are really a great distance apart. Venus is closer to the sun than the Earth, whereas Jupiter is about five times farther from the sun than the Earth is.
If we look east, we see Mars about 45 degrees above the horizon in the constellation Leo. Mars shines in bright orange-red and is only 6 or 7 degrees from the star Regulus. On March 3, Mars was opposite the sun as seen from Earth, which made it due south at midnight standard time. This was the closest we will be to Mars for the next two years. (You may remember from an earlier column that this is why we now have a spacecraft on its way to Mars.) You can still find Mars close to due south at 11 p.m. daylight time (midnight standard time). The earth is now passing Mars because of its closer/faster orbit, so Mars will appear smaller and dimmer over the coming months.
You will have to wait until about 9 p.m. for Saturn to rise in the east. It reaches its highest point in the sky due south about 2 a.m. Saturn is about 6 degrees to the left of the star Spica and about twice as bright. Even with a small telescope you can see Saturn's rings, which are tilted about 14 degrees from our view.
This is where the planets will be at 8 p.m. tonight; because of the earth's rotation during the night they will move with the stars slowly to the west. Jupiter will set first, a little after 10 p.m. Because the Earth also moves in its orbit around the sun, the stars and planets will be a little farther west at the same time subsequent nights.
See if you can find these planets in the sky. Watch them move relative to the stars and to each other. Become a planet hunter!
Marty Scott is the astronomy instructor at Walla Walla University, and also builds telescopes and works with computer simulations. He can be reached at firstname.lastname@example.org.